Canal Regulation

By

Dr. Dipankar Roy

Canal Regulation

The water which enters into the main canal from the river to be divided into different branches and distributaries, in accordance with the relative urgency of demand on different channels

This process of distribution is called Regulation

Canal Regulation

• Canal Falls
• Canal Regulators (Head Regulator and Cross Regulator)
• Canal Escapes
• Metering Flumes etc.
• Canal Outlets and Modules

What is Canal Fall?

Canal fall is a solid masonry structure which is constructed on the canal if the natural ground slope is steeper than the designed channel bed slope. If the difference in slope is smaller, a single fall can be constructed. If it is of higher then falls are constructed at regular suitable intervals.

Location of Canal Falls

Location of canal fall depends upon the following factors

Types of Canal Falls and their Importance

The important types of falls which were used in olden days and those which are being used in modern days are described below:

• Ogee falls
• Rapids
• Stepped falls
• Trapezoidal notch falls
• Well type falls
• Simple vertical drop falls
• Straight glacis falls
• Montague type falls
• English falls or baffle falls

Ogee Falls

Ogee curve is the combination of convex and concave curves. So, Ogee fall consists of both convex and concave curves gradually.

This gradual combination helps to provide smooth transition of flow and also reduce the impact.

If the canal natural ground surface is suddenly changed to steeper slope, ogee fall is recommended for that canal.

Stone pitching is provided in the upstream and downstream of the fall.

Ogee fall

Rapid Canal Falls

Rapid fall consists a long sloping glacis. It is constructed if the available natural ground surface is plane and long. For this, a bed of rubble masonry is provided and it is finished with cement mortar of 1:3 ratio. To maintain the slope of bed curtain walls are provided at both upstream and downstream. Rapid falls are high priced constructions.

Rapid fall

Stepped Canal Falls

As in the name itself, stepped fall consist vertical steps at gradual intervals. Stepped fall is the modification of rapid fall. It is suitable for the canal which has it upstream at very high level as compared to downstream. These two levels are connected by providing vertical steps or drops as shown in figure.

Stepped Canal Falls

Trapezoidal Notch Canal Falls

In case of trapezoidal notch falls, a high crested wall is built across the channel and trapezoidal notches are provided in that wall. Trapezoidal falls are very economical and suitable for low discharges. Now a days this type of falls are using widely because of their simplicity and popularity.

Trapezoidal Notch Canal Falls

Well Type Canal Falls

Well type falls are also called as syphon drop falls. In this case, an inlet well with pipe at its bottom is constructed in upstream.

The pipe carries the water to downstream well or reservoir.

If the discharge capacity is more than 0.29 cumecs then downstream well is preferred otherwise reservoir is suitable.

Vertical Well Drop

Pipe Drop

Simple Vertical Drop Falls (Sarda Type fall)

Simple vertical drop fall or sarda fall consists, single vertical drop which allows the upstream water to fall with sudden impact on downstream.

The downstream acts like cushion for the upstream water and dissipate extra energy. This type of fall is tried in Sarda Canal UP (India) and therefore, it is also called Sarda Fall.

Sarda Type Falls

Straight Glacis Canal Falls

This is the modern type of construction, in which a raised crest is constructed across the canal and a gentle straight inclined surface is provided from raised crest to the downstream.

The water coming from upstream crosses the raised crest and falls on inclined surface with sufficient energy dissipation.

Straight Glacis Canal Fall

Straight Glacis Canal Fall

Montague Type Canal Falls

Montage fall is similar to straight glacis fall but in this case the glacis is not straight.

It is provided in parabolic shape to introduce the vertical component of velocity which improves the energy dissipation to more extent.

English or Baffle Canal Falls

In this case, straight glacis fall is extended as baffle platform with baffle wall. This is suitable for any discharge.

The baffle wall is constructed near the toe of the straight glacis at required distance in designed height.

The main purpose of the baffle wall is to create hydraulic jump from straight glacis to baffle platform.

Canal regulators

Canal regulators

• These include the cross regulator and the distributary head regulator structures for controlling the flow through a parent canal and its off-taking distributary.

• They also help to maintain the water level in the canal on the upstream of the regulator.

• Canal regulators, which are gated structures, may be combined with bridges and falls for economic and other considerations, like topography, etc.

Offtaking canal alignment

a) Smooth offtake

b) Both inclined to original flow

c) Parent canal flows straight with reduced width

Functions of Head Regulator

• To regulate or control the supplies entering the off-take channel
• To control silt entry into the off-take channel
• To serve as a meter for measuring discharge

Functions of Cross Regulator

• To effectively control canal irrigation system

• When the water level in the main channel is low, it helps in heading up on the U/S and to feed the off-take channels to their full demand in rotation.

• They help in absorbing fluctuations in various sections of the canal system, and in preventing the possibilities of breaches in the tail reaches.

• Cross regulator is often combined with a road bridge so as to carry the road which may cross the irrigation channel near the site of the cross regulator. It is also usually combined with a fall; when it is called fall regulator.

Canal regulators

• To prevent excessive entry of silt deposition at the mouth of the off-take, the entry angle should be kept to between and .

• For the hydraulic designs of cross regulators, one may refer to the Bureau of Indian Standard code IS: 7114-1973 “Criteria for hydraulic design of cross regulators for canals”

• The water entering in to the off-taking distributary canal from the parent canal may also draw suspended sediment load.

Silt vanes

IS: 6522-1972 “Criteria for design of silt vanes for sediment control in off-taking canals”

Silt vanes, or King’s vanes, are thin, vertical, curved parallel walled structures constructed of plain or reinforced concrete on the floor of the parent canal, just upstream of the off-taking canal.

The height of the vanes may be about one-fourth to one-third of the depth of flow in the parent canal.

Silt vanes

IS: 6522-1972 “Criteria for design of silt vanes for sediment control in off-taking canals”
The thickness of the vanes should be as small as possible and the spacing of the vanes may be kept about 1.5 times the vane height.

To minimize silting tendency, the pitched floor on which the vanes are built should be about 0.15 m above the normal bed of the parent channel.

Groyne walls or curved wings

IS: 7871-1975 “Criteria for hydraulic design of groyne wall (curved wing)
These are curved vertical walls, also called Gibb’s groyne walls, which project out in to the parent canal from the downstream abutment of the off-taking canal.

The groyne wall is provided in such a way that it divides the discharge of the parent canal in proportion of the discharge requirement of the off-taking canal with respect to the flow in the downstream parent canal.

Groyne walls or curved wings

IS: 7871-1975 “Criteria for hydraulic design of groyne wall (curved wing)
The groyne wall extends upstream in to the parent canal to
cover 3⁄4 to full width of the off-take. The proportional distribution of flow in to the off-taking canal is expected to divert proportional amount of sediment, too.

Skimming platforms

IS: 7880-1975 “Criteria for hydraulic design of skimming platform for sediment control in off-taking canal"

A skimming platform is an RCC slab resting on low height piers on the bed of the parent canal, and in front of the off-taking canal, and in front of the off-taking canal

This arrangement actually creates a kind of low tunnel at the bed of the parent canal, which allows the sediment moving along its bed to pass through downstream.

Skimming platforms

IS: 7880-1975 “Criteria for hydraulic design of skimming platform for sediment control in off-taking canal"

The floor of the off-taking canal being above the level of the platform thus only takes suspended sediment load coming along with the main flow in the parent canal.

A skimming platform arrangement is suitable where the parent channel is deep (about 2m or more) and the off-take is comparatively small.

Skimming platforms

Canal Escapes

Canal Escapes

These are structures meant to release excess water from a canal, which could be main canal, branch canal, distributary, minors etc.

Though usually an irrigation system suffers from deficit supply in later years of its life, situations that might suddenly lead to accumulation of excess water in a certain reach of a canal network may occur due to the following reasons:

• Wrong operation of head works in trying to regulate flow in a long channel
  resulting in release of excess water than the total demand in the canal system
  downstream.

• Excessive rainfall in the command area leading to reduced demand and
  consequent closure of downstream gates.

• Sudden closure of control gates due to a canal bank breach.

Weir or surface escapes

Sluice or surplus escapes These are gated escapes with a very low crest heigh.
Hence, these sluices can empty the canal much below its full supply level and at a very fast rate.

In some cases, these escapes act as scouring sluices to facilitate removal of sediment.

Sluice or surplus escapes

These are constructed in the form of weirs, without any gate or shutter and spills over when the water level of the canal goes above its crest level

Canal Escapes

• The locations for providing escapes are often determined on the availability of suitable drains, depressions or rivers with their bed level at or below the canal bed level so that any surplus water may be released quickly disposed through these natural outlets.

• Escapes may be necessary upstream of points where canals takeoff from a main canal branch.

• Escape upstream of major aqueducts is usually provided.

• Canal escapes may be provided at intervals of 15 to 20km for main canal and at 10 to 15km intervals for other canals.

Metering Flumes

Metering Flumes

• A metering flume is an artificially flumed (narrowed) section of the channel which can be utilized for calculating the discharge in the channel

• The normal upstream section of the channel is narrowed by masonry walls with a splay of 1:1 to 2:1 to a rectangular section called throat.

• From throat, the channel is slowly diverged so as to attain its normal section by means of masonry wings with a splay of 2:1 to 10:1.

• More gradual the convergence and divergence, less will be the loss of head in the flume.

Metering Flumes

Venturi Flume

A venturi flume consists of a gradually contracting channel leading to throat and a gradually expanding channel leading away from it.

Stilling wells are provided for measuring head at the entrance and the throat.

Venturi Flume

If is the difference of head between the
two wells, then the discharge:

where, varies from 0.95 to 1.0, area of entrance, area of throat

Standing wave Flumes

it is designed in such a way as to form a hydraulic jump or standing wave on the downstream portion of the flume. In this case, the level of the downstream portion of flume is kept lower than throat level to form hydraulic jump. Hence, the discharge through the throat section depends only on the upstream head.

Standing wave Flumes

In this case, single head measurement at the throat section is enough to calculate the discharge of channel. Stilling well is provided at the throat to measure the head. Discharge through free flow Venturi flame is calculated by using the following expression :

Where, = Coefficient of discharge of Venturi flume (generally varies from 0.92 to 1.0)

= width of throat section

= Head measured in stilling well.

Parshall Flume

Parshall flume is a modified version of venturi flume. Some modifications are made in venturi flume to change the flow conditions from sub critical to supercritical and those modifications are as follows :

• Increase in throat length
• Reduction in angle of convergence of inlet walls
• Reduction in angle of divergence of outlet walls
• Drop in elevation through the throat of the flume

Design of Parshall flume can be done as either free flow parshall flume or submerged parshall flume like in venturi flume.

Cut-throat Flume

A cut-throat flume consists of a gradually contracting channel section followed by a gradually expanding channel section. Throat section is eliminated in case of cut-throat flumes. The bottom surface of a cut-throat flume is flat and horizontal.

The construction of cut-throat flume is easier compared to other types of flume since it requires horizontal floor and flat metal sheets and, also, there is no need of throat section.

Canal Outlets or Modules

Canal outlets, also called farm turnouts in some countries, are structures at the head of a water course or field channel. The supply canal is usually under the control of an irrigation authority under the State government.

Since an outlet is a link connecting the government owned supply channel and the cultivator owned field channel, the requirements should satisfy the needs of both the groups.

Since equitable distribution of the canal supplies is dependent on the outlets, it must not only pass a known and constant quantity of water, but must also be able to measure the released water satisfactorily.

Canal Outlets or Modules

Various types of canal outlets have been evolved from time to time but none has been accepted as universally suitable.

It is very difficult to achieve a perfect design fulfilling both the properties of flexibility as well as sensitivity because of various indeterminate conditions both in the supply channel and the watercourse of the following factors:

• Discharge and silt

• Capacity factor

• Rotation of channels

• Regime condition of distribution channels, etc.

Requirements of a Good Module

• The module should fit well to the decided principles of water distribution.

• Should be easy to construct and fabricate.

• Should work efficiently at a small working head.

• Should be cheap as they are required in large numbers.

• It should draw its fair share of silt.

• It should not be interfered by the cultivators

Types of Module

• Non-modular outlets

• Semi modules or Flexible modules

• Rigid module or Modular outlets

Non-modular outlets

• These outlets operate in such a way that the flow passing through them is a function of the difference in water levels of the distributing channel and the watercourse.

• Hence, a variation in either affects the discharge.

• These outlets consist of regulator or circular openings and pavement. The effect of downstream water level is more with short pavement.

Example: Open sluice and Drowned pipe outlet

Open masonry sluice

Pipe Outlet

Semi-modules or Flexible modules

The discharge through these outlets depend on the water level of the distributing channel but is independent of the water level in the watercourse so long as the minimum working head required for their working is available.

Example: Pipe outlet, Venturi flume, Open flume and Orifice semi-module

Rigid Modules or Modular outlets

• The discharge through modular outlets is independent of the water levels in the distributing channel and the watercourse, within reasonable working limits.

• This type of outlets may or may not be equipped with moving parts.

• Though modular outlets, like the Gibb’s module, have been designed and implemented earlier, they are not very common in the present Indian irrigation engineering scenario.

Gibbs Module

Minimum modular head

The minimum difference between upstream and downstream water levels, which is required to be maintained so as to enable the module to pass the design discharge is known as Minimum modular head or Minimum modular loss.

Efficiency of an outlet

It may be defined as the ratio of the head recovered to the head put in. Lesser is the head required for functioning of the outlet; more efficient the outlet will be. Efficiency is the a measure of the conservation of head by the outlet.

Drowning Ratio

It is the ratio of the depth of water level over crest on the downstream of the module to the depth of water level over crest on the upstream of the module. In case of a weir type outlet, the efficiency is the same as the drowning ratio.

Modular limit and Modular range

The modular limits are the extreme values of any one or more variables, beyond which an outlet becomes incapable of acting as a module or semi-module. The range between the lowest and the highest limiting values of various such factors is known as modular range.

Flexibility

The ratio of the rate of change of discharge of the outlet to the rate of change of discharge of the distributary channel.

where,

= Discharge passing through outlet

= Discharge in the distributary channel

Flexibility

Since a change in the water depth of the distributary would result in equal change in the head working on the outlet ,

Proportionality

• The outlet is said to be proportional when the rate of change of outlet discharge is equal to the change of the channel discharge.

• The outlet is proportional when F=1

• For a proportional outlet,

Setting

The ratio , i.e. the ratio of the depth of the sill level of the outlet below the FSL of the distributary, to the full supply depth of the distributary, is known as setting.

Sensitivity

Defined as the ratio of the rate of change of discharge through the outlet to the rate of change of water level of the distributary,referred to normal depth of the channel.

For modular outlets, discharge is fixed, and hence the sensitivity is zero

For a flexible module, where the discharge through the outlet is independent of the watercourse and depends only upon the level of the distributary, a gauge can be fixed and calibrated so as to indicate its reading when . Thus,

Relation between Sensitivity and Flexibility

thus,

since , or

• Greater is the variation in the discharge of an outle for a given rise or fall in water surface level of the distributary, the larger is the sensitivity of the outlet.
• The sensitivity of non-flexible module is zero.

Codes

• IS 4410 : Part 15 : Sec 4 : 1977 Glossary of terms relating to river valley projects: Part 15 Canal structures Section 4 Regulating works

• IS 6522 : 1972 Criteria for design of silt vanes for sediment control in offtaking canals

• IS 6531 : 1994 Canal Head Regulators - Criteria for Design

• IS 6936 : 1992 Guide for location, selection and hydraulic design of canal escapes

• IS 7114 : 1973 Criteria for hydraulic design of cross regulators for canals

Codes

• IS 7495 : 1974 Criteria for hydraulic design of silt selective head regulator for sediment control in offtaking canals

• IS 7880 : 1975 Criteria for hydraulic design of skimming platform for sediment control in offtaking canal

• IS: 6936-1992 (reaffirmed 1998) “Guide for location, selection and hydraulic design of canal escapes”

• IS: 7114-1973 “Criteria for hydraulic design of cross regulators for canals”

End of section